void main(List<String> args) {
  int a = 123;
  print(a);
  String? username = "张三"; //---？可空类型
  username = null;
  print(username!.length); //---！非空断言
  List<String>? l1 = ['张三'];
  l1 = null;
  print(l1);

  String? getData(str) {
    if (str != null) {
      return str;
    }
    return null;
  }

  print(getData(null));
}
